Background Calibration of Bit Weights in Pipelined-SAR ADCs Using Paired Comparators

Abstract

This brief presents a background calibration technique for pipelined successive-approximation-register (pipelined SAR) analog-to-digital converters (ADCs), which resolves the errors from capacitor mismatches and inaccurate interstage gain errors. The dither signal is injected in the capacitor digital-to-analog converter (DAC), while its residue voltage increment is neutralized through paired comparators with opposite polarity offsets, thereby relaxing the design requirement of the residue amplifier. While one of the comparators is generating the residue signal, the other one is detecting the signal range and helping to obtain the bit weights. This brief also introduces the circuit design of paired comparators with opposite offsets. The background calibration technique is verified in a 5b + 8b pipelined SAR ADC. Simulation results show that the spurious-free dynamic range (SFDR) and the signal-to-noise and distortion ratio (SNDR) are improved from 54.5 to 94 dB and 49 to 68.9 dB, respectively. The mean value of the voltage swing increment is 34 mV with noise sources, offset, gain error, and capacitor mismatches.